Project Summary/Abstract The overall goal of this project is to determine how output from the basal ganglia influences cerebral cortical activity in the processes of decision making, motor planning, and movement execution. The studies will employ mice as the best suited species in order to bring modern optogenetic and genetically encoded sensor technologies to bear on this critical gap in our understanding of brain function. In aim 1 we address the impact of basal ganglia output on network activity in cortex across sensory and motor areas. To this end will use genetically encoded calcium sensors selectively expressed in thalamic neurons receiving input from the basal ganglia (BGT) to record the pattern of activation of these thalamic axons in cortex with wide-field imaging. We will further image the resulting activation or inhibition of these thalamic terminals in cortex upon optogenetic manipulations of basal ganglia output activity in quietly awake mice and mice performing a forced choice left/right licking task. In a second study under aim 1 we will use genetically encoded voltage sensors to image the postsynaptic activation of specific cortical cell types upon optogenetic basal ganglia output manipulations. The expected outcome of these studies is that we will have characterized the impact of basal ganglia modulated thalamic activity on cortical network activation. In aim 2 we will address the question of how these network effects are mechanistically achieved at the cellular and subcellular level. We hypothesize that the input of BGT, which is primarily restricted to superficial cortical layers, will result in the activation of non-linear dendritic properties of pyramidal cell dendrites such as calcium or NMDA spikes. To address this hypothesis we will use simultaneous 2-photon calcium imaging in thalamic terminals and cortical dendrites in the context of our behavioral task. In a second study we will use whole cell recordings in behaving mice in conjunction with optogenetic basal ganglia output manipulations to determine the balance of excitatory and inhibitory effects converging on pyramidal cells as a consequence of basal ganglia activity. Finally, in aim 3 of our proposed research we will use detailed biophysical neural modeling to construct a thalamo-cortical network model that can replicate the observed physiological responses to basal ganglia output manipulations. On the subcellular level, we will use this model to determine the specific synaptic input strengths and voltage-gated ion channel types in pyramidal neuron dendrites that are required to explain observed responses. On the network level we will use the model to search through a large number of optogenetic basal ganglia output manipulations to identify candidate stimulus patterns that indicate specific mechanisms at work. We will then employ these patterns in our recordings to test model predictions and come to a better understanding of network interactions resulting from basal ganglia activity. Overall, we expect that our work will result in a much improved mechanistic understanding of basal ganglia thalamo-cortical signal transmission, and how dysfunction of this pathway contributes to symptoms in basal ganglia disorders such as Parkinson?s disease.